As I've stated in some previous threads, I generally have 3 windows open with 20 or more tabs open in each. Most of the time they stay in the original order, 1, 2, 3.

Every once in a while, they will suddenly shift to 1, 3, 2 or maybe 2, 1, 3, etc. I don't really know why, but it seems to happen when a script on one tab hangs momentarily. It does not happen all the time however, even when a script hangs. Once it does, I can't re-order them by dragging on the Task Bar for example or by any other method. More of an annoyance than a problem, but I'd like to know what's happening.